The present invention relates to a process for production of water-absorbing polymer structures, the water-absorbing polymer structures obtainable by this process, water-absorbing polymer structures, a composite, a process for production of a composite, a composite obtainable by this process, chemical products comprising water-absorbing polymer structures or a composite, as well as the use of water-absorbing polymer structures or of the composite in chemical products.
Superabsorbers are water-insoluble, crosslinked polymers, which are capable of absorbing, and retaining under a given pressure, large quantities of aqueous liquids, in particular body fluids, preferably urine or blood, by swelling and forming hydrogels. Because of these characteristic properties, these polymers principally find applications by incorporation into sanitary articles, such as, for example, baby diapers, incontinence products, or sanitary napkins.
Superabsorbers which are presently commercially available are mostly crosslinked polyacrylic acids or crosslinked starch acrylic acid graft polymers, in which the carboxyl groups are partially neutralized with sodium hydroxide or potassium hydroxide.
For aesthetic and environmental reasons, there is an increasing tendency to make sanitary articles ever smaller and thinner. In order to guarantee a consistent overall retention capacity of the sanitary article, this demand can only be accommodated by reduction of the proportion of voluminous fluff. This means that further tasks fall to the superabsorber in respect of transport and distribution of liquids, which can be summarized as permeability properties.
By permeability in superabsorber materials is understood the capacity, in the swollen state, to transport and uniformly distribute penetrating liquids within the swollen gel. This process occurs via capillary transport through spaces between the gel particles. Liquid transport through the swollen superabsorber particles itself follows the laws of diffusion and is a very slow process which plays no role in the distribution of liquid in the use situation of the sanitary article. For superabsorber materials which cannot accomplish a capillary transport because of lacking gel stability, a separation of the particles from each other, with avoidance of the gel blocking phenomenon, is effected by the embedding of these materials into a fiber matrix. In diaper constructions of newer generations, there is only little or even no fiber material in the absorber layer to support liquid transport. The superabsorbers used here must correspondingly have a sufficiently high stability in the swollen state for the swollen gel to have a sufficient amount of capillary spaces, through which liquid can be transported.
In order to obtain superabsorber materials with high gel stability, on the one hand, the degree of crosslinking of the polymer can be increased, which necessarily results in a reduction of the swell capability and of the retention capacity.
Furthermore, methods for post-treating of the surface of polymer particles for improvement of the superabsorber properties can be used. Surface treatments known from the state of the art are, for example, the post-crosslinking of the water-absorbing polymer structure at the surface, the bringing into contact of the surface with inorganic compounds, or the post-crosslinking of the surface in the presence of inorganic compounds.
DE-A-100 16 041 achieves a restoration of the gel permeability of water-absorbing polymers, which has been damaged by mechanical action, by post-treating such a polymer after a post-crosslinking with a solution of at least one salt of an at least trivalent cation.
WO-A-98/48857 discloses superabsorbent polymers having an improved Gel Bed Resiliency, which was obtained by the dry mixing of the polymers with a multivalent metal salt and the subsequent contacting of the mixture with a binding agent. A mixing of this type with inorganic, fine particulate substances has disadvantages, such as, for example, demixing or dust.
WO-A-98/49221 describes the rewetting of water-absorbing polymers with an aqueous solution of an additive comprising a mono- or multivalent metal salt after a heat treatment, which leads to polymers with improved processability.
The disadvantage of the water-absorbing polymer structures known from the state of the art is, however, among others, that absorbent structures, such as, for example, absorbent cores in diapers, comprise as absorption agents previous polymer structures with higher absorption capacity, which, upon sudden entry of particularly large amounts of body fluids, as can occur, for example, with adult diaper wearers or upon wetting of a diaper by an older child suffering from Enuresis nocturna (“bed-wetting”), are not capable of fully absorbing and further distributing within the absorbent structure the amount of fluid, in particular under a pressure caused by the lying diaper wearer.
In general, the invention has the object of overcoming the disadvantages arising from the state of the art.